For over fifty years, cancer has remained the second leading cause of death in the United States, accounting for over 25% of the deaths in the population. More than one million cases are diagnosed each year, resulting in over 500,000 deaths (American Cancer Society, 2001). Nanotechnology may offer new options for both diagnosis and treatment of cancer. A new nanoparticle-based approach to cancer therapy has been under investigation in our laboratory. Near infrared-absorbing nanoparticles are injected intravenously and allowed to accumulate in the tumor, due to the leaky vasculature and/or targeting, followed by illumination of the animal with near infrared (NIR) light. NIR light is not appreciably absorbed by tissue components, allowing deep penetration without damage to normal tissues. Using a class of NIR-absorbing nanoparticles called gold-silica nanoshells, we have demonstrated complete tumor ablation and long term survival of animals without tumor regrowth. We have recently begun in vitro studies with two other classes of NIR- absorbing nanoparticles - gold-gold sulfide nanoshells and gold nanorods. In vitro, we have been able to achieve much more rapid heating with these two newer types of nanoparticles due to their higher absorption and lower scattering (order of heating, nanorods, gold-gold sulfide nanoshells, gold-silica nanoshells), and thus believe that they have the potential to be more effective at lower doses in cancer therapy than gold- silica nanoshells. However, the sizes of these three classes of nanoparticles varies widely. Gold-silica nanoshells are approximately 100 nm in diameter, gold-gold sulfide nanoshells approximately 50 nm, and gold nanorods 20 nm. Thus, biodistribution of the particles will be quite different and may drastically affect therapeutic efficacy. We propose to evaluate biodistribution, biocompatibility and tumor ablation efficacy of these NIR-absorbing nanoparticles. The optimal particle formulation will be further examined in a model of medulloblastoma. [unreadable] [unreadable]